1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission which is in particular mounted on an automobile, and which more particularly relates to a line pressure control device and the structure of a primary regulator valve and a cutback valve.
2. Description of the Prior Art
Conventionally, a hydraulic control device for an automatic transmission mounted on an automobile has a cutback valve. The cutback valve has a control chamber to which hydraulic pressure is applied at a speed greater than the second speed, an input port to which a throttle pressure is applied and an output port which supplies the input pressure to the throttle valve as a cutback pressure.
When operating at the first speed, because hydraulic pressure is not applied to the second brake hydraulic servo, the input port and output port of the cutback valve are disconnected, so that the cutback valve applies no cutback pressure to the throttle valve. Under this condition, because the throttle valve is supplied with no cutback pressure at the spool return side, the throttle valve makes the line pressure a comparatively high throttle pressure, and this high throttle pressure is applied to the primary regulator valve, so that the line pressure is set comparatively high, and a high clutch engaging force is generated corresponding to a high load torque at the time of starting. At the second speed, hydraulic pressure is applied to the second brake hydraulic servo, so that the input port and the output port of the cutback valve are switched so as to be connected, and the cutback pressure is applied to the throttle valve. Under this condition, the throttle valve is supplied with the cutback pressure at the spool return side, so that a comparatively low throttle pressure is generated, and this low throttle pressure is applied to the primary regulator valve. As a result, the line pressure is set to be low, so that the engaging force of clutch and brake is low to prevent shock during shifting.
With regard to the above line pressure control, as explained, the cutback pressure is applied to the throttle valve. Consequently, in the throttle valve at a top end of the spool, a spring from a downshift plug is applied, while at a bottom end of the spool, a return spring is applied. Furthermore, the throttle valve has three areas on the spool: an area to regulate the line pressure to the throttle; a first step area where a feedback pressure caused by the throttle pressure is applied; and a second step area where the cutback pressure is applied. Due to this arrangement, the spool has a three-step structure.
Such three-stage structure makes the throttle valve more complicated because the throttle valve normally has a downshift plug and a throttle cam having a power to assist mechanism. Thus, forming of the valve requires intricate work and is laborious, and the cross section area of the step areas may be small. Consequently, the hydraulic response is not sufficient, and furthermore, the structure is complicated and the average diameters of the spool may be small, so that valve sticking caused by foreign materials may easily occur.
At the second speed range, the cutback valve is in the connected condition at the second speed, so that the line pressure is set to be comparatively low. Consequently, if a vehicle attempts to get out of a tire rut or start on a snowy road, the line pressure is comparatively low even at the second speed starting, so that the engaging force of the frictional engaging elements is sufficient, and efficient starting is not performed.
Generally, with regard to an automobile having an automatic transmission, when a selection lever is operated from the neutral to the drive D or the reverse R range, the line pressure is applied to a hydraulic servo of one of several frictional engaging elements, one of the frictional engaging elements (a forward clutch, a direct clutch or a first and reverse brake) are suddenly engaged, and so-called selecting shock may occur.